Radiation-cured “radcure” coatings are formulated materials that are cross-linked or cured using energy from electron beams or from ultraviolet or visible light radiation. Radcure coatings facilitate high-throughput production with low thermal energy requirements. Typically, cure is initiated throughout the bulk of the coating, but more particularly nearest to the source of the irradiation (unless there are atmospheric oxygen inhibition effects). The performance of most radcure materials, whether protective or adhesive, relies on strong bonding to the surface.
The most widely used radcure materials contain photoinitiators and other low molecular weight species. During the radiation cure, these materials tend to be excluded from the bulk polymerization phase, diffusing either to the exposed surface or the interface between the substrate and the polymerized bulk. When low molecular weight species diffuse to the interface, there is a general tendency to reduce or compromise adhesion. Another undesirable consequence is the diffusion of the low molecular weight species to the exposed or exterior surface, where they accumulate and plasticize the bulk resin, and tackiness is often observed.
There is an obvious need to maximize interfacial bonding in both adhesives and protective coatings and, in particular, it is of great value to do this in radcure compositions. Initiating cure at the polymer-substrate interface rather than in the bulk or surface closest to radiation source has intrinsic advantages because primary reactions would initiate at the interface and non-reactive and low molecular weight species would be forced to diffuse from the highly crosslinked area. Similarly, a cure initiated at the exposed surface would reduce the tendency to form “tacky” films. Further, there are instances in which it would be desirable to control photoinitiation, particularly in environments in which exposure to even low levels of light prior to formal cure has deleterious effects.
Premature radiation cure (prior to application to a substrate) is a problem not only under storage conditions, but in developing optimum performance in adhesive bonding applications. A composition that undergoes a radiation cure that can be switched on by exposure to surface hydroxyl groups or atmospheric moisture is a “dual exposure system” which precludes premature cure by random exposure to radiation. In the case of surface hydroxyl activation, maximum bonding to the surface is ensured by activating polymerization (cure) at the surface.